Among infrared heating apparatus whose application is expected in a wide range of fields such as cooking and heating of foods and drying of coated products, etc., a surface combustion burner is known as one of techniques which employ as a heat source thereof a gas fuel that is low in cost and high in calories.
The surface combustion burner is such that the heat energy of a combustion gas, which is largely taken out by convection in the case of the ordinary combustion, is efficiently converted to a radiant heat and it is designed so that a mixture or a premix of air and a gas fuel is supplied from one side of a permeable sheet member ( hereinafter referred to as a burner diaphragm ) and the mixture is burned in the surface layer portion on the other side of the burner diaphragm, thus heating the surface layer portion itself of the burner diaphragm and thereby causing it to discharge the radiant heat. Thus, in the surface combustion burner the combustion of the gas is maintained in a condition where a flame is brought into close contact with the surface of the burner diaphragm or entered into the surface layer portion and the radiant heat is radiated from the flame and the burner diaphragm surface layer portion heated to a red hot state.
With the conventional surface burners, those of the type in which a porous sintered metal sheet or sintered ceramic sheet is used as a raw material for its burner diaphragm have already been put in practical use in some fields such as cooking utensils and others using a fiber mat composed of metal or ceramic fibers sintered in layer form have been studied vigorously. These surface combustion burners are advantageous in that in addition to the fact that a radiant heat can be obtained with high efficiency, a stable combustion is possible which is not dependent on the external environments such as wind and temperature. Particularly, since the burner diaphragm composed of a mat made by sintering stainless steel fibers can be formed to have a complicated surface shape and its strength is excellent and since the realization of a high-porosity structure makes it possible to easily manufacture a burner which is large in area, low in pressure loss, high in combustion degree and high in power output density and which is relatively inexpensive, its application to such uses as a heating apparatus at an outdoor job site and the baking and drying of automobile painting is expected.
FIG. 3 is a schematic diagram showing the construction of an infrared heater used at an outdoor job site as an example of a surface combustion burner apparatus using a burner diaphragm made of a stainless steel fiber mat, and its principal part including the burner diaphragm is shown in section.
In FIG. 3, the burner diaphragm m is composed of a stainless steel fiber mat of 5 mm thick which is made by forming stainless steel ( JIS-SUS 316 ) long fibers of 20 .mu.m in diameter and about 50 mm in length into a mat shape and sintering the long fibers together. With this burner diaphragm m, its surface layer portion ml forms a gas combustion zone during the operation of the apparatus and this gas combustion zone is a radiant heat radiation portion.
Here, a fuel gas supply system including a gas nozzle N, a solenoid valve SV and a fuel gas bomb T and an air supply system including an air blower or fan F are connected to a burner proper K to which the burner diaphragm m is attached. In addition, a spark electrode S for ignition purposes is arranged in opposition to the lower end of the burner diaphragm m so that when its switch is operated, a controller C not only brings the solenoid valve SV and the blower F into operation but also applies a spike-like high voltage between the spark electrode S and the burner diaphragm m thus producing a discharge spark and thereby igniting the gas-air mixture at the surface of the burner diaphragm m. These component members are mounted on a movable base B which is equipped with wheels.
Then, when the switch of the controller C is operated so that the heating apparatus is started, the solenoid valve SV is opened causing the injection of the fuel gas through the nozzle N and also the blower F is started thus supplying air whereby inside the burner proper K the resulting mixture of the fuel gas and the air flows toward and passes through the burner diaphragm m thereby soaking out to the outside through the surface layer portion ml. On the other hand, a spark is produced between the spark electrode S and the burner diaphragm m across which a high voltage has been applied so that the air-gas mixture soaking out to this portion is ignited and a flame is rapidly propagated all over the surface of the burner diaphragm thereby starting the burning operation.
At this time, in order that this surface combustion burner may effect an efficient combustion, the amount of gas supply and the amount of air supply must be controlled exactly. In other words, the ratio of the amount of gas supply to the amount of air supply ( the mixture ratio ) is made substantially equivalent to a chemical reaction stoichiometric amount ratio and also the flow rate of the gas-air mixture passing through the burner diaphragm m is selected to be in such a range that the flame does not get off the surface of the burner diaphragm m. As a result, the stable combustion is maintained in the surface layer portion ml of the burner diaphragm and the surface layer portion ml is heated red hot, thereby radiating a radiant heat in an amount substantially dependent on the surface temperature of the surface layer portion ml.